The present invention relates to a method for simulating deviations in surface appearance of plastics parts caused by a forming process of plastics parts according to claim 1, and a respective computer system, computer program and computer program product.
The surface appearance of plastics products is a very important topic in the plastics industry. A major criteria of plastics product quality is the surface appearance. The surface quality of plastics parts is influenced by the surface structure, the forming process parameters and by the polymer material itself. Although a lot of efforts to improve surface appearance are done, the results are not always satisfying. The so called tigerstripes phenomenon of the surface appearance has been object of the scientific world for many years.
The plastics part consists of a polymer material of specific geometric design and may comprise thermoplasts and/or thermosetting plastics comprising duroplastics and elastomeric polymer materials. Plastics parts are produced by a forming process.
The forming process is a process where the polymer raw material is formed to specific plastics parts under expenditure of forming energy. In most of the cases the polymer raw material has the form of pellets. Different forming processes are common in the plastics industry: e.g. injection molding including special types of injection molding processes: like gas assisted injection molding, co-injection molding, backside molding, compression molding and any combination of injection molding and compression molding, polymer extrusion, blow molding and foaming.
A plastics particle or polymer particle is defined as a volume portion of the plastics part. The sum over volume of all plastics particles of a plastics part coincides with the volume of the plastics part.
The surface of a plastics part is defined as the total face (surface) of the plastics part in any details. Due to the fact that plastics parts are generally thin, the ratio between plastics part face to plastics part volume is quite high. This fact also emphasises the importance of the surface quality of plastics parts.
The surface appearance of plastics parts is a very important issue of quality in the plastics industry. The surface appearance is determined on one hand by physical material properties and by inherent properties of the polymer and polymer morphology of very small sized dimension such as color pigments, crystallinity, constituents, kind of molecules. On the other hand the surface appearance is determined by the surface structure of the plastics part, which is the negative copy of the cavity surface of the tool, where the plastic melt is injected. Grain and roughness of the cavity surface are the main structure parameters in terms of surface quality of the plastics parts. It is evident that parameters of the forming process influence the surface appearance of plastics parts.
The surface layer defines a thin layer of polymer material, which influences the surface appearance of the part. Different surface appearance effects are determined by different physical constitutions and therefore the thickness of the surface layer may vary with the effect examined. It is defined, that the volume particles positioned exactly on the surface of the plastics part are part of the surface layer, but polymer particles beneath may also be part of the surface layer.
The “tigerstripes” phenomenon is a common problem in the plastics industry and describes a specific surface defect. Tigerstripes, as known in the plastics industry, describe a visible periodic inhomogenity in surface gloss. Mostly these are alternating dull (or rough) and glossy (or smooth) areas on the surface of injection molded or extruded plastics parts, which surface should be glossy (or smooth) all over.
The thermoplastic polymer may consist of polyolefins, such as polyethylene (PE) and polypropylene (PP), but also of polyamides (PA), polycarbonates (PC), acrylonitrile-butadiene-styrene (ABS), polymethyl methacrylate (PMMA) or polyetherimide (PEI) and compounds and blends of thermoplastic polymers. Compounds are polymers or blends of polymers with every kind of constituents like talc, minerals, glass, rubber and pigments. In addition polymer compounds are equipped with additivation packages.
The simulation of the forming process is common in the plastics industry and is based on the continuum mechanics combined with the method of Finite Elements and/or Finite differences. Finite difference computer based solving algorithms (software) to simulate the forming process are available on the market, such as software of Moldflow Corporation, see e.g. EP 1 218 163 B1. The content of EP 1 218 163 B1 is herewith incorporated as disclosure of the invention of this patent application.
EP 1 218 163 B1 discloses a method of modelling the injection of a fluid (plastic melt) into a mold three-dimensionally. The basis for such a model are the equations for the conservation of mass, momentum and energy and can be expressed in the form shown in equations (3) to (6) in EP 1 218 163 B1 or in the form of the e.g. Navier-Stokes equations (see also page 9, lines 31-32 of EP 1 218 163 B1):
                              ∂          p                          ∂          t                    +              ∇                  ·                      (                          ρ              ⁢                                                          ⁢              u                        )                                =    0                                            ∂            u                                ∂            t                          +                              (                          u              ·              ∇                        )                    ⁢          u                    =                                    -                          1              ρ                                ⁢                      ∇            p                          -                  ∇          ϕ                +                              μ            ρ                    ⁢                                    ∇              2                        ⁢            u                                ,                  ⁢                            ρ          ⁡                      (                                                            ∂                  ɛ                                                  ∂                  t                                            +                              u                ·                                  ∇                  ɛ                                                      )                          -                  ∇                      ·                          (                                                K                  H                                ⁢                                  ∇                  T                                            )                                      +                  p          ⁢                      ∇                          ·              u                                          =      0.      whereasρ is the density of the fluid,ν is the velocity,ρ is the pressure,Φ is the potential energy per unit mass,T is temperature,μ is the coefficient of shear viscosity;ε is the internal energy per unit mass,KH is the thermal conductivity.
Since the conservation of energy is included in the model, thermal effects can be taken into account during the forming process, and thus for example the thermal effects of phase changes from liquid to solid state can be considered accordingly. In EP 1 218 163 B1 it is said that the following heat transfer mechanisms can be modelled: convection (from the incoming melt), conduction (out of the mold wall), and viscous dissipation (which is related to the thermal energy produced by shearing within the flowing plastic material). Also other mechanisms, such as compressive heating effects (due to heat generated by compression) and cooling effects form decompression (see p. 4, lines 8-11) are to be considered.
The above cited equations describing the flow of the liquid thermoplastic material are solved by a discrete numerical procedure, such as a Finite Element Method, or any other suitable method, see e.g. page 8, line 14-16 of EP 1 218 163 B1. Using the Finite Element Method, the mold cavity volume is divided into single finite elements (finite volumes) and the field variables (such as pressure, temperature, velocity) and the material properties (such as viscosity) are computed iteratively for a node point of a finite element and then interpolated within the element volume.
However, EP 1 218 163 B1 is silent with respect to detecting and predicting surface defects of the plastics part.